Patients with neuropsychiatric disorders, affective disorders, and dementias often display abnormal brain oscillations, as measured by electrical potentials on the scalp. One oscillatory band of interest is the theta band (4-10 Hz), which is disrupted in conditions ranging from schizophrenia to Alzheimer's. Behavioral tests reveal that patients with these disorders often display impaired working memory, a process known to be associated with elevated theta oscillations in healthy subjects. Are disrupted theta oscillations the underlying cause of these impairments, or is their appearance merely correlated with cognitive deficits? Routes toward potential therapies should be influenced by the answer to this question. Extensive studies in rodents provide further evidence for the importance of theta in cognitive tasks, but-until now-it was impossible to disrupt theta rhythms without changing the properties of the entire circuit. Today, optogenetic techniques allow us to causally manipulate the activity of genetically defined cell populations on the timescale of milliseconds. I will harnes this ability to disrupt theta-band activity in the hippocampus, a critical hub for theta generation Specifically, I will block the output of the hippo- campus at particular phases of theta, which wil allow me to observe trial-to-trial effects on working- memory performance. If I observe a phase-specific behavioral deficit, it will indicate that the segregation of spikes within an individual teta cycle is, in fact, important for behavioral guidance. If no phase- specific disruption is observed,it would suggest that theta is not important for short-term behavioral guidance, at least in the dorsal CA1 region. Instead, this result would favor the role of theta in long-term memory storage, something that could be tested in future experiments. This proposal represents the first attempt to interact with the hippocampus on the timescale of theta. To do so, I will need to receive training on the most efficient ways to read out the state of the hippocampus online, in order to implement phase-specific stimulation. I already have experience with optogenetics and electrophysiology from my work in the laboratory of Christopher Moore, now at Brown University, but so far all of my experiments have involved open-loop stimulation. The lab of my sponsor, Matthew Wilson, has a wealth of experience with closed-loop stimulation especially that related to disrupting oscillatory activity in the hippocampus. A Kirschstein-NRSA Fellowship lasting two years would provide the support necessary to fund my training and bring my experiments to completion. I hope these experiments will help set a precedent for combining optogenetics and closed-loop feedback, which represents a powerful approach to studying the relationship between abnormal brain rhythms and abnormal cognition.